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mr2racer 6th August 2012 12:12 AM

Dynaco low Power Amp
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Can anyone tell me how the cathodes of the 12AX7'S are grounded? It seems the only path to ground is through the second tube grid? Or does the ground float? If I knew the name of the circuit I might be able to find a description somewhere.

20to20 6th August 2012 12:29 AM

Put your pencil on the cathode and trace the line backward through the resistors. This is a K biased tube so it is not directly grounded like a fixed biased tube would be.



kevinkr 6th August 2012 12:33 AM

Through the feedback connection to the secondary.

mr2racer 6th August 2012 01:03 AM

if it is grounded through the secondary how does that not put DC into the outputs? Is it just that the DC would rather go to ground and bypass the voice coils?

AH! I don't have the ground from the speaker common to the chassis! That's why the preamp tubes aren't working. Should that connection go to the star ground?

kevinkr 6th August 2012 01:11 AM

The cathode current is miniscule, (a couple of mA max) so the voltage developed across the dcr of the secondary winding is no more than a couple of mV at most.

rmyauck 6th August 2012 02:32 AM

I believe that's a Floating Paraphase circuit.


bear 6th August 2012 03:41 AM

AC is grounded through the very large electrolytic... fwiw.


DF96 6th August 2012 01:00 PM

Yes, the input stage looks like a floating paraphase. The OPT secondary should be grounded to the same point as the input ground, so the input signal and the negative feedback are referenced to the same potential.

kevinkr 6th August 2012 05:02 PM


indianajo 7th August 2012 12:58 AM

This is a portion of the dynaco SCA35 circuit which Greg Dunn covers on his website.

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